1. Field of the Invention
The present invention relates to a carburetor of the type in which a fluid nozzle is disposed in a venturi to blow a jet into the fuel from a fuel nozzle thereby to impart a kinetic energy thereto so that the flow rate of the fuel to emanate from the fuel nozzle into an intake pipe may be controlled to accurately, stably and smoothly control the mixing ratio between the intake air and the fuel.
2. Description of the Prior Art
In a carburetor C according to the prior art, as shown in FIG. 1, the intake air flowing through an intake pipe 1 is mixed with a fuel under an emulsion condition, which is sucked and introduced through both such a main nozzle 3 as is opened into a venturi 2 arranged in the intake pipe 1 and such slow and idle ports 5a and 5b as are opened in the vicinity of a throttle valve 4 arranged downstream of the venturi 2 to form a combustible mixture, thereby to prepare the air-fuel mixture, which is fed to a combustion chamber 6.
In the prior art caburetor C, moreover, the air-fuel ratio is determined by the fuel flow rate, which is set by the vacuum pressure in the venturi 2 or at the throttle valve 4, to the flow rate of the intake air, which is set by the opening of the throttle valve 4. Thus, the main and slow jets 7 and 8 in a fuel passage are so selected as to prepare a preset air-fuel ratio. This air-fuel ratio is, however, liable to be fluctuated in accordance with the running conditions (such as the R.P.M., load or temperature) of an engine E.
As the means in the conventional carburetor C for controlling the air-fuel ratio, therefore, there have been proposed a concept, in which bleed air is mixed in advance and the bleed flow rate of the air to be additionally mixed is controlled by air bleeds 9a and 9b upstream of the main nozzle 3 and the slow port 5a, and another concept in which their controls are effected by varying the effective areas of the main nozzle 3, the main jet 7 and the slow jet 8.
According to the former concept, however, the flow modes such as the bubble flow, the slag flow or the piston flow are varied by the flow rate of the bleed air. Moreover, these variations in the flow modes are invited by the flow rate of the additional bleed air. As a result, even if it is intended to control the air-fuel ratio by mixing the additional bleed air under the condition having the instable flow mode, the flow rate itself of the additional bleed air becomes so instable that the air-fuel ratio can not be effected to a satisfactory extent.
In fact, the Inventors have confirmed that the timely fluctuations in the bleed air flow rate are so high as to invite practical problems. In view of these, according to the air bleed flow rate control, pulsations take place in the fuel, which is to be sucked and introduced from the main nozzle 3 or the slow port 5a, thereby to invite practical problems in the responsiveness and controllability. As a result, the mixture of a proper air-fuel ratio cannot be prepared, and the supply of the improper mixture to the combustion chamber 6 results in the fluctuations in the output of the engine E thereby to invite the deteriorations in the engine performance and drivability and the emission of noxious contents in the engine exhaust gases.
Moreover, the aforementioned air bleeds 9a and 9b function partly to effect the excellent atomization of the fuel and partly to improve the transient characteristics between the main system for supplying the main nozzle 3 with the fuel and the slow system for supplying the slow and idle ports 5a and 5b with the fuel.
As shown in FIGS. 2 and 3, more specifically, the improvements in the transient characteristics are performed such that the condition, under which the air-fuel ratio temporarily becomes excessively high to provide a lean mixture in the range where the air flow rate is gradually increased so that the discharge of the fuel from the main system is started, is shifted to the lower or richer side of the air fuel ratio by the actions of the air bleeds 9a and 9b.
In FIG. 2, the ordinate and the abscissa respectively show the fuel flow rate (Gf) and the air flow rate (Ga), wherein line CN shows Ga/Gf is constant, lines T, M and S respectively show the total fuel flow rate, the fuel flow rate through a main system and the fuel flow rate through a flow system.
On the other hand, in FIG. 3, the ordinate and the abscissa respectively show the air-fuel ratio (A/F) and the air flow rate (Ga), wherein lines D and I respectively show A/F in the case of no air bleed and A/F in the case of with air bleed.
In practice, however, the air bleeds 9a and 9b don't play such an important role for the fuel atomization in the carburetor C but is rather effective in the improvements in the transient characteristics. Thus, the air bleeds 9a and 9b improve the air-fuel ratio in the transient phase but generate the aforementioned pulsations in the fuel discharge thereby to induce the fluctuations in the fuel discharge in the ranges other than that of the transient characteristics with the resultant problem that the control of the air-fuel ratio becomes difficult.
According to the aforementioned latter concept, the requirement for drastic changes in the conventional carburetor and the complexity in construction to vary the effective areas of the main nozzle 3 and the main and slow jets 7 and 8 requires the highly precise machining, and to further finely control the diameters relating thereto is technically difficult at present.
According to one of the means for solving the problems thus far described, atmospheric pressure is introduced into the venturi to establish the pressure change so that the flow rate to be sucked and introduced into the venturi may be controlled and improved. However, the several series of experiments and analyses conducted by the Inventors have confirmed that a proper control of the air-fuel ratio over a wide range cannot be expected in the least.